Earl of Derby (i/ˈdɑːbi/DAR-bi) is a title in the Peerage of England. The title was first adopted by Robert de Ferrers, 1st Earl of Derby under a creation of 1139. It continued with the Ferrers family until the 6th Earl forfeited his property toward the end of the reign of Henry III and died in 1279. Most of the Ferrers property and, by a creation in 1337, the Derby title, were then held by the family of Henry III. The title merged in the Crown upon Henry IV's accession to the throne. It was created again for the Stanley family in 1485.

Lord Derby's subsidiary titles are Baron Stanley of Bickerstaffe in the County Palatine of Lancaster (created 1832), and Baron Stanley of Preston, in the County Palatine of Lancaster (1886). The 1st to 5th Earls also held an earlier Barony of Stanley, created for the 1st Earl's father in 1456 and currently abeyant; the 2nd to 5th Earls held the Barony of Strange created in 1299, currently held by the Viscounts St Davids; and the 7th to 9th Earls held another Barony of Strange, created in error in 1628 and currently held independently of other peerages.

Several successive generations of the Stanley Earls, along with other members of the family, have been prominent members of the Conservative Party, and at least one historian has suggested that this family rivals the Cecils (Marquesses of Salisbury) as the single most important family in the party's history. They were at times one of the richest landowning families in England.

Arms of Ferrers Earls of Derby: Vaire or and gules a bordure argent of horseshoes sable

The Ferrers canting coat of arms shows six black horseshoes (French: fer de cheval) on a silver background. The family of Ferrers was descended from Henry de Ferrers, 1st Count of Ferrières,[2] Lord of Longueville[disambiguation needed], Normandy, and a Domesday Commissioner; he built Tutbury Castle and Duffield Castle and had large holdings in Derbyshire as well as 17 other counties. The Ferrers, lords of the barony of Ferrieres in Normandy, were accompanied to England by three other families who were their underlords in France: the Curzons (Notre Dame-de-Courson), the Baskervilles (Boscherville) and the Levetts (Livet-en-Ouche).

Robert de Ferrers, 1st Earl of Derby, 2nd Earl of Ferrières (1062–1139) was created Earl of Derby by King Stephen in 1138 for his valiant conduct at the Battle of Northallerton. He was married to Hawise de Vitre and died in 1139.

The large estates which were taken from Robert in 1266 were given by Henry III to his son, Edmund Crouchback; and his son, Thomas, Earl of Lancaster also called himself Earl Ferrers. In 1337 Edmund’s grandson, Henry of Grosmont (c. 1299–1361), afterwards Duke of Lancaster, was created Earl of Derby, and this title was taken by Edward III's son, John of Gaunt, who had married Henry’s daughter, Blanche. John of Gaunt’s son and successor was Henry Bolingbroke, who acceded to the throne as Henry IV in 1399. The title Earl of Derby merged into the Crown.

His eldest son and heir apparent George Stanley, Lord Stanley (commonly called Lord Strange), married Joan Strange, 9th Baroness Strange and 5th Baroness Mohun, and was summoned to the House of Lords as Lord Strange in right of his wife. Lord Derby was succeeded by his grandson Thomas, the eldest son of Lord Strange. He had already succeeded his mother as tenth Baron Strange and sixth Baron Mohun. He married Anne Hastings, daughter of Lord Hungerford and Hastings. The second Earl's son Edward became the 3rd Earl. He notably served as Lord High Steward at the coronation of Queen Mary of England in 1553 and was Lord Lieutenant of Cheshire and Lancashire.

Lord Derby was married four times. His second wife Dorothy Howard, daughter of Thomas Howard, 2nd Duke of Norfolk, supplied his heir Henry, the fourth Earl. He served as Ambassador to France and was one of the peers at the trial of Mary, Queen of Scots, in 1586. Lord Derby married Margaret Clifford, daughter of Henry Clifford, 2nd Earl of Cumberland, and his wife Eleanor, younger daughter of Charles Brandon, 1st Duke of Suffolk, and his wife Mary Tudor, daughter of King Henry VII. Both Lord Derby's sons succeeded to the earldom. The eldest son Ferdinando, the fifth Earl, was summoned to the House of Lords through a writ of acceleration in his father's junior title of Baron Strange in 1589. He also built Leasowe Castle, probably as an observation platform for watching horse races on the nearby sands. Lord Derby married Alice Spencer, but was without male issue. He died under mysterious circumstances and some have claimed that he was poisoned in order to prevent him from staking a claim to the throne of England through his maternal grandmother. On his death the baronies of Stanley, Strange and Mohun fell into abeyance between his three daughters.

He was succeeded in the earldom by his younger brother William, the sixth Earl. He was Lord Lieutenant of Lancashire and Cheshire and purchased from his nieces their claims on the Isle of Man. William married Elizabeth de Vere daughter of Edward de Vere, 17th Earl of Oxford. Their son James succeeded to the earldom on his father's death. In 1628, during his father's lifetime, he was summoned to the House of Lords through a writ of acceleration as Lord Strange, as it was believed that his father held this title. When it was discovered that this was a mistake, the House of Lords decided that there were two baronies of Strange, the original 1299 creation and the new, 1628 creation. James was a staunch Royalist. In 1643 he moved to the Isle of Man and established it as a Royalist stronghold. He was beheaded by the Parliament forces. His wife was Charlotte de la Trémouille, daughter of Claude de la Trémoille, Duc de Thouars, is known as the heroine who defended Lathom House in 1644 and the Isle of Man in 1651.

Their son Charles became the 8th Earl. He served as Lord Lieutenant of both Cheshire and Lancashire. Lord Derby married Dorothea Helena Kirkhoven, daughter of Baron Rupa of the Netherlands. They had two sons who both succeeded to the earldom. The 8th Earl's eldest son William Richard George became the 9th Earl. He was also Lord Lieutenant of Cheshire and Lancashire. He married Elizabeth Butler, daughter of Thomas Butler, Earl of Ossory, and sister of James Butler, 2nd Duke of Ormonde. He had two daughters and one son. He outlived his son and on his death in 1702 the barony of Strange fell into abeyance between his daughters. He was succeeded in the earldom by his younger brother, James, the tenth Earl. He was a politician and served as Chancellor of the Duchy of Lancaster and as Captain of the Yeomen of the Guard. Like many of his predecessors he was also Lord Lieutenant of Cheshire and Lancashire. In 1732 he succeeded his great-niece as 6th Baron Strange. Lord Derby was childless and on his death in 1736 the male line of the second Earl died out. He was succeeded in the barony of Strange, which could be passed on through female lines, by his first cousin once removed, James Murray, 2nd Duke of Atholl.

The earldom was inherited by his distant relative Sir Edward Stanley, 5th Baronet, of Bickerstaffe, a descendant of a younger brother of the second Earl, who became the 11th Earl of Derby (see below for earlier history of the Baronetcy). He had previously represented Lancashire and Parliament and after he succeeded in the earldom he served as Lord Lieutenant of Lancashire. Lord Derby married Elizabeth Hesketh. His residence was Bickerstaffe Hall near Ormskirk, Lancashire. The 11th Earl's younger brother was the Hon. and Rev. John Stanley, Rector of Bury Parish Church 1743–1778. Edward's eldest son, James, Lord Stanley, was commonly called Lord Strange. Edward outlived James (who died in 1771) and was succeeded by James' son Edward, the 12th Earl. He held political office as Chancellor of the Duchy of Lancaster and also founded the Epsom Derbyhorse-race. Lord Derby married Lady Elizabeth Hamilton, daughter of James Hamilton, 6th Duke of Hamilton. His second marriage was to a Drury Lane actress, Elizabeth Farren.

The 12th Earl's first marriage produced his heir Edward, the thirteenth Earl. He represented Preston and Lancashire in the House of Commons and in 1832, two years before he succeeded his father, he was raised to the peerage in his own right as Baron Stanley, of Bickerstaffe in the County Palatine of Lancaster. Lord Derby was also a natural historian and his zoological collections founded Liverpool Museum. He was also a patron of the arts, especially of the poet Edward Lear who wrote The Owl and the Pussycat for the Earl's children. He was married to Charlotte Hornby. In 1844, he had a church built on the Knowsley Estate, St. Mary the Virgin, where several Stanleys found their final resting place.

The Stanley Baronetcy, of Bickerstaffe in the County Palatine of Lancaster, was created in the Baronetage of England in 1627 for Edward Stanley. He was the great-grandson of the Hon. Sir James Stanley, of Cross Hall, Lathom, younger brother of the second Earl of Derby. This branch of the family is known as the "Stanleys of Bickerstaffe". Sir Edward Stanley's great-grandson, the fourth Baronet, represented Preston in Parliament. His son, the aforementioned fifth Baronet, succeeded as eleventh Earl of Derby in 1736. For further history of the baronetcy, see above.

Several Earls of Derby are buried in St. Mary's Church, Knowsley. Others are buried in the Derby Chapel at Ormskirk Parish Church.

The Barons Stanley of Alderley are members of another branch of the Stanley family. They are descended from the Hon. Sir John Stanley, third son of the first Baron Stanley. Also, Edward Stanley, 1st Baron Monteagle, was the younger son of the first Earl of Derby.

^"Ferrer" is Norman French and means "to bind with iron" or " to shoe a horse" (cf. farrier). Ferrières in Normandy, the hometown of the de Ferrers family, is claimed by some to be an important centre for ironwork

Coward, Barry. The Stanleys, Lords Stanley, and Earls of Derby, 1385–1672: The Origins, Wealth, and Power of a Landowning Family. (Remains Historical and Literary Connected with the Palatine Counties of Lancaster and Chester, 3d series, vol. 30) Manchester University Press (for the Chetham Society), 1983.